This disclosure relates to a two wire, two-way paging system between master and remote stations, and more particularly to such a paging system in which direction of transmission is sensed and thereafter controlled by the master station.
Various systems are known which have as their purpose detection of transmission in a two-way paging system so that control may be obtained by one station or the other in the paging system and one transmission will therefore not interrupt another. Moreover, it is desirable to have a master control station which may capture and hold control of the paging system as long as a speaker at the master station continues to talk into the system. One such scheme involves frequency discrimination, wherein a common voice frequency, such as 700 Hertz for example, is filtered out in a very narrow band from the signals originating at the remote station. A transmission detector sensing apparatus is provided to receive the master transmission, which is sensitive only to the 700 Hertz in the master transmission, and the presence of such a frequency causes the control system to place the master in control. The detector is insensitive to transmissions from the remote station, because the selected voice frequency of 700 Hertz is not present. This type of system is proper in theory, but is difficult to attain practically, since voice notch filters for very narrow band filtering are not available with steep enough upper and lower band definition slopes. Moreover, harmonics of the direction sensing frequency might energize the detector providing false directional indication.
Another scheme utilizes voltage level sensing for directional indication. Signals originating at the master station are amplified so that signals normally originating there will exceed a predetermined voltage level, such as 3 volts for example. The system attempts to insure that the transmission signals originated at the remote station are always below the predetermined voltage level of about 3 volts. Therefore, master station signals appearing above the 3 volt level will capture and hold control of the system for the master station. Remote station signals below the 3 volt level will allow the remote station to assume control and transmit information from the remote to the master station. Problems arise when signals at the remote station exceed the predetermined voltage level of 3 volts, in this example, due to high sound energy levels. The remote station therefore may gain control in the middle of a transmission when loud or sharp sounds are made at the remote station, thereby cutting off transmission. Moreover, equipment at the master station must not present an impedance to the system outside the range of approximately 300-900 ohms.
A practical sensing and control system is necessary for a two-way paging network which is not dependent upon transmission level or frequency discrimination for transmission direction sensing, and which allows equipment at the master station to present an impedance coupled to the paging lines which may assume a value within a wide impedance range.